The present invention relates to power shovels and, more particularly, to power shovels having a dipper adapted for excavating earthen material. Specifically, the present invention relates to latches for dipper doors.
Large electric mining rope shovels utilize a digging attachment consisting of a stationary boom with a combination handle/dipper structure which mounts on the boom and actively crowds and hoists into a bank in order to dig with and fill the dipper. The dipper is rigidly connected to the handle. After digging through a bank face and filling with material, the dipper is lifted and the upper portion of the shovel is rotated relative to the lower portion of the shovel so that the dipper is positioned above a waiting dump truck. The operator then activates a tripping mechanism that opens a dipper door latch on the bottom of the dipper and allows a door to swing down and open. The contents of the dipper are then dumped into the truck bed.
The heavy dipper door is pivotally mounted on a lower end of the dipper. The conventional mechanical latch mechanism secures the door in its closed position and, when released, allows the door to open under the force of gravity. The conventional latch mechanisms, as shown in FIGS. 2 and 3, typically include a trip wire 1 or cable assembly which has one end adapted for control by a power shovel operator and another end connected to a moveable latch lever 2 which is generally located on the dipper door 3. The latch lever is typically coupled to a slidable rod or latch bar 4 that is selectively engaged in a latch keeper opening 5 (see FIG. 3) in a front wall 6 of the dipper body of the dipper. The dipper door 3 is held closed when the latch bar 4 is within the latch keeper opening 5. The dipper door is caused to open by tripping the trip cable 1 which moves the latch lever 2 which causes the latch bar 4 to slide away from the latch keeper opening 5 and disengage the latch keeper opening 5, whereby the dipper door 3 will open under its own weight plus the weight of any material contained within the dipper body. The latch lever 2 provides significant mechanical leverage to slide and pull the latch bar 4, which is under very high load.
Normally, the door is thereafter closed by swinging the dipper in such a direction so as to cause the dipper door to move by inertia towards its closed position until the latch bar reengages the latch keeper. More particularly, the latch bar is forced away from the dipper front wall by contacting the latch keeper wall with a sloping surface that causes the latch bar to push up, and then the latch bar drops into the slot by gravity, locking the door.
This is a simple device including mainly two bars pulling on each other with dry, sliding friction contact that has worked well for many years; however, with the increase in dipper size over the past few years, its reliability has become compromised because dry sliding friction levels have increased under higher contact loads making behavior less predictable.
There are maintenance problems with this system, especially when it is used with 100-ton or larger payload dippers. Latch bars and related operating equipment are a significant part of dipper maintenance cost.
The maintenance problems include broken pull chains and clevis. The pull chain is the chain that connects a tugger rope to the latch bar lever. The trip ropes have to be replaced constantly as they become frayed and strands break. The snatch block (sheave) and bracket of the trip mechanism break or have to be replaced on a regular basis. The tugger motor, gearing and drum take a continuous beating causing repeated failures. And lastly, the latch bar has to have shimming added, and this is a constant ongoing maintenance issue that in the long run becomes labor intensive and costly.
The amount of tension required to trip or move a latch bar on a fully loaded 120-ton payload dipper nearly exceeds the mechanical ability of this system. Larger motors are usually the solution, but the net result is a very high cost maintenance area.
Another problem with conventional mechanical latch closure mechanisms is the tendency for such mechanisms to quickly wear out and require replacement in only a short period of time. Each time the slidable latch bar engages the latch keeper or the like, the tip of the slidable latch bar naturally wears down. In many conventional latch mechanisms, the slidable latch bar is only moved about a half inch to about an inch in order to allow the dipper door to open. Thus, only a very small portion, i.e., the tip, of the slidable latch bar comes into contact with the latch keeper. Since the latch bar is under very high load and the contact area is very small, the tip experiences very high contact forces that cause an accelerated rate of wear. As the tip of the slidable latch bar wears down over time, it becomes possible for the dipper door to prematurely open before the power shovel operator is ready for the dipper door to open. This, as can be appreciated by those skilled in the art, can create a hazardous and unsafe condition if the power shovel is not properly maintained.
To account for this wear, the latch bar length of engagement with the latch keeper must be frequently adjusted by adding or removing shims to the latch lever pivot mechanism 6 (see FIG. 2). This requires the lifting of the heavy latch lever and latch bar to insert and remove the shims, usually with the assistance of a crane or forklift. Thus, conventional latch closure mechanisms exhibit operational shortcomings that must be addressed with more frequent, hazardous, and costly maintenance activities.
Examples of other past dipper latch approaches include Hilgeman U.S. Pat. No. 2,544,682 that illustrates a pivoting latch with a primary locking mechanism and a secondary latch, and Brown Jr. U.S. Pat. No. 6,467,202 that illustrates a dipper door pivoted and held by a linkage mechanism.